First Test for Early HIV Infection

The U.S. Food and Drug Administration has announced the approval of Aptima®, a new assay that can be used to aid the detection of HIV in those who may be newly infected with the virus and cannot yet depend on the results of antibody testing. However, even if Aptima testing yields a positive result, standard antibody testing must still be conducted to confirm the diagnosis of HIV infection.

The Aptima HIV-1 RNA Qualitative Assay, developed by San Diego-Based Gen-Probe Incorporated, is a nucleic acid test (NAT) for the detection of HIV in blood samples. It is intended to be used as an aid in the diagnosis of HIV infection, including acute or primary infection, before the appearance of antibodies to HIV.

A traditional HIV diagnosis is based on testing for antibodies to the virus by enzyme immunoassay (EIA), with positive results confirmed using supplemental antibody tests such as Western blot or immunofluorescence assays (IFA). Although the sensitivity of HIV antibody detection has increased in the last few years with the availability of new assays, a window period between infection and antibody detectability still exists. Following a recent exposure to HIV, it usually takes up to three months for the antibody response to reach detectable levels using EIA testing. Until three months after a possible exposure has occurred, a negative test result using EIA cannot be considered totally reliable.

The newly approved test may provide earlier diagnosis of infection because it detects nucleic acid (RNA) of HIV in a blood sample. In other words, the test looks for the virus itself, not the antibodies produced by the immune system.

The test, however, is not meant to be used as a stand-alone assay for the diagnosis of HIV infection. A positive Aptima test result should be viewed as an unconfirmed test result, indicating probable infection. It should be followed up later with traditional EIA antibody testing to confirm infection with HIV.

Aptima may also be used as an additional test to confirm HIV infection in someone whose specimen is repeatedly reactive for HIV antibodies. This is important because the Western blot can, in some instances, be difficult to interpret and may not always provide a conclusive positive test result. According to the FDA, the Aptima test can be used instead of the traditional Western blot test or IFA for confirming HIV infection when the screening test result for HIV antibodies is positive.

Aptima is different from quantitative HIV assays, better know as viral load tests. Qualitative assays, such as Aptima, detect the presence of viral genetic material and give a "yes-no" answer as to whether the virus is present. In contrast, quantitative assays, such as Roche's Amplicor®, are used to estimate the amount of genetic material present in a sample. Aptima is not approved for quantitative use. Similarly, viral load tests – although sometimes used to diagnose early HIV infection – are not approved for this purpose.


Source: Tim Horn, October 6, 2006 (AIDSmeds)


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